The play to be performed is usually between two or three months away from the opening night and you tend to rehearse for three hours, three times a week say on a Monday Wednesday and Friday night. Nearer the performance week the cast may rehearse for some time on a Sunday - perhaps four hours. Is this enough? Usually not, and when I was working fulltime I would spend my breaks and lunch hour rehearsing privately as well as at home when I wasn't out rehearsing. So clearly putting in plenty of time going over the lines is important.
I have found that getting them learnt as early as possible really helps the final performance because once the script is out of your hand, and really, only then, can the character be worked on and your confidence in the words and part, grow.
A lot of actors would say that your physical place on the stage is very important for learning the lines as your brain associates where you are positioned with the lines delivered. I would agree. If you go marching off in a different direction during a performance or rehearsal you risk confusing the association and therefore the line. The same happens if another person isn't in the place they normally are.
In regard to the lines themselves I have used a variety of tricks from writing them out in big letters to looking at the lines themselves and how the words are spelt and linking them. Eg: The water was falling down all over him, Brian. Here I have connected the w in the word water and was and in my head I would also visualise what I was reporting. I also find that playing with the lines and saying them out loud and in a daft and very much exaggerated voice to myself can help. Sometimes I pace around the room at speed to build up an energy whilst saying the lines. Finding the rythmn in the speech and looking for light and shade helps too.
Once the lines are getting in my head I put the book down and try and do as much as I can without it. Naturally it is very important to be able to remember the other characters lines as well - or at least the gist of what they are saying and the cues. The cue is the block of words at the end of the previous line.
On top of all this is the useful habit of knowing the storyline backwards so you have an holistic grasp of the play. Then of course it is vital to know your character and build upon the interpretation in conjunction with your fellow actors and the director.